Meanwhile, a CDC study found that tests for another common STD -- gonorrhea -- miss one in three infections among men who have sex with men.

Those findings alone are troubling enough. But the picture becomes bleaker against the background of a surging HIV epidemic.

"This increase in syphilis represents a major concern for the health of gay and bisexual men," Kevin Fenton, MD, director of the CDC's HIV/AIDS center, said at a news conference. "STDs increase HIV transmission. And if a person is already HIV infected, syphilis can increase viral load [a measure of HIV activity in the body]."

Fenton announced the new STD data at this week's National STD Prevention Conference in Chicago.

Syphilis Surges Again

The syphilis numbers are shocking because just a decade ago, the U.S. was on the verge of eliminating the disease. Now 3.7 out of every 100,000 Americans -- and 6.4 out of every 100,000 American males -- carry the infection, which can cause blindness, brain damage, and death if untreated.

Rates among men are six times higher than rates among women. This, Fenton said, is largely because of huge increases in syphilis infections among men who have sex with men. About 60% of all recent U.S. syphilis infections occurred in these men.

African-Americans continue to be disproportionately affected by syphilis. Syphilis rates are six times higher for African-American men than among white men, and 13 times higher for African-American women than for white women.

Gonorrhea Test Misses Gay/Bi Men

Syphilis and HIV aren't the only diseases spread mainly by unsafe sex. Gonorrhea is also on the rise. One factor, a CDC study suggests, is that doctors miss more than one in three gonorrhea infections in HIV negative gay and bisexual men.

The problem seems to be that doctors aren't testing men who have sex with men in all three body sites where they are likely to be infected: the penis (urethra), the rectum, and the throat.

CDC researcher Kristen C. Mahle, MPH, and colleagues found that doctors miss more than one in three rectal gonorrhea infections, one in four throat infections, and one in 10 urethral infections in men who have sex with men.

Part of the problem is that the most effective tests for gonorrhea are nucleic acid amplification tests or NAATs. These tests are FDA approved for testing for gonorrhea infections in the urethra. But they are not approved for testing the throat or rectum.

Julius Schachter, PhD, of the University of California, San Francisco, reported to the conference that NAAT tests work very well in screening men's throats and rectums for gonorrhea and for chlamydia.

John Douglas, MD, director of the CDC's STD center, said the CDC would work with the FDA to gain approval of expanded use of NAATs.

"NAATs detect twice as many gonorrhea and chlamydia infections as bacterial culture tests," Douglas said at the news conference. "FDA clearance of NAATs would allow for their widespread use in medical settings."